This disclosure relates generally to an interface device and method for supplying gas flow for subject breathing. The disclosure also relates to an apparatus for supplying anesthetic agent to the interface device.
On general inhalation anesthesia anesthetic drugs are used to keep patients experiencing surgical operation relaxed, motionless, unconscious, and free from pain. The anesthetic drugs interfere on the central nervous system for these effects. On inhalation anesthesia the anesthetic drugs are delivered with breathing gas to lungs where they get diffused to patient blood circulation. This circulation further carries the drug to the effect site in brains.
Inhalation anesthesia drugs are halogenated hydrocarbons that are delivered on administration site as liquids. These liquids are very volatile with vapor pressure at room temperature varying between 20-95 kPa. These liquids are vaporized for patient breathing in anesthesia vaporizer. The most common inhalation anesthesia drugs are isoflurane, sevoflurane and desflurane. These have replaced the use of their predecessors, halothane and enflurane.
For various reasons preference of the agent to be used varies between patients and clinics. Conventional vaporizers are heavy devices to enclose thermal energy for the cooling caused by liquid vaporization. The devices are positioned for convenient use of the output concentration dial embedded on the device. Because of the weight and elevated position at anesthesia system, their installation may be laborious for clinical personnel. For this reason the anesthesia machines are equipped with functionality that enables easy selection of the agent to be used among those readily connected to the system. Anesthesia system accommodates therefore often two or three sockets to connect the vaporizer. Advantage of separate, anesthesia system independent module for vaporization provides is to have functional redundancy against vaporizer failure.
In operation, vaporizer receives fresh gas, which is a mixture of oxygen, nitrogen, and nitrous oxide, and completes that with required percentage of the anesthetic drug vapor. On conventional passive vaporizers the completion occurs with vaporization of the liquid agent respective to its vapor pressure. Alternate technologies are active vaporization of liquid to gas and control of this gas flow, or controlling agent liquid flow and mixing to fresh gas stream. The prepared fresh gas is then delivered from vaporizer outlet to anesthesia breathing system for patient.
Arising from the operational principle of vaporizers, if two vaporizers would be active in anesthesia system they would both deliver the required concentration to the gas stream. Both of these drugs would then get delivered for patient breathing and circulation to effect-site both drug causing their effect resulting to doubled strength of the anesthesia effect. Clinically such situation is challenging to manage and may be dangerous. Therefore vaporizer constructional standards require mechanisms that prevents simultaneous opening of the vaporizers.
In vaporizer failure situation for provision of un-interrupted inhalation anesthesia delivery, the anesthesia system must have means to isolate the failing vaporizer from the pneumatic circuitry and continue the anesthesia with another vaporizer connected to the anesthesia system, or replace the damaged unit with an operational one. To achieve isolation, the components potentially endangered to fail are advantageously positioned to the exchangeable module and the anesthesia system has reliable means to isolate the vaporizer from the rest of the system. Isolating the vaporizer from anesthesia system pneumatic circuitry when the vaporizer is inactive also ensures that vaporizer not selected for use does not leak vapor for patient breathing. Current anesthesia systems have a valve at the vaporizer inlet connector and outlet connector. These valves can be switched to position allowing gas flow through vaporizer or bypassing the vaporizer.
One of the most sensitive components that renders a vaporizer unusable is a leak of the seal between the vaporizer and the anesthesia system. Damage may occur when removing and installing the vaporizer on the anesthesia system. Identifying the presence of such a leak is difficult in the middle of delivering anesthesia. Therefore the vaporizer connection is advantageously evaluated at the anesthesia system testing performed regularly. In such testing, identification of the site for observed leak is advantageous to ease problem solving. Valves on the anesthesia system isolating the seal between the vaporizer and anesthesia system provides advantage of controlled testing of the leak in this connection. In such testing, one vaporizer at a time is connected to the pneumatic circuitry and the circuit leak is analyzed by pressurizing the circuit.
One particular undesired failure mode of anesthesia system is occlusion of the fresh gas line. When in use fresh gas is guided to the vaporizer though an inlet valve and out from the vaporizer through an outlet valve. When one of the valves makes a proper connection, but the other fails to make the connection, fresh gas occlusion occurs. Such a failure to open one of the valves may occur if that valve gets stuck or the valve actuator is broken.
Modern anesthesia systems are electronic other than vaporizers, where the mechanical actuation (including connection valves) and passive vaporization still dominate. These devices lack what electronics provide, such as automatic therapy data storage to patient records, automated device diagnostics, drug usage measurement, monitoring remaining drug level and external control of the desired output from anesthesia system.
However, external electronic control of the vaporizer from anesthesia system allows positioning of the vaporizers away from the prime user interface area, as well as providing anesthesia automation. Provision of electrical energy for vaporization miniaturizes size and weight, enabling ergonomic development of anesthesia systems. Such electronically controlled vaporizers must also have electronic control of the vaporizer connection valves to facilitate automatic vaporizer activation on user request for agent output and for automatic testing.